1. Field of the Invention
The present invention relates to a process for the preparation of shaped, electrically conductive articles from silicon powder by sintering the powder while being pressed or sedimented in a mould.
2. The Prior Art
Silicon powder while being can be sintered by heating the powder pressed or sedimented in a mould to sufficiently close to the melting point. For silicon a temperature of 1370.degree.-1405.degree. C. is necessary at a given melt temperature of 1410.degree. C. (internal report, Technical University of Eindhoven, January 1976, from J. Krop; C. Greskovich and J. H. Rosolowski, J. Am. Ceramic Society, 59, 336-343 (1976)). When, however, the powder particles are very fine, i.e., &lt;0.2 .mu.m, at temperatures from 1250.degree. C. sintering phenomena can be already observed. At higher temperatures, namely above 1390.degree. C., larger particles of up to 3 .mu.m can be sintered after addition of boron (see U.S. Pat. Nos. 4,040,848 and 4,040,849).
Contrary to many other substances which show sintering phenomena at much lower temperatures with respect to the melting point, the covalent character of silicon must be considered responsible for the necessary high temperature and the required very fine particles. The preparation of the electrically-conductive shaped articles by sintering of silicon powder is under these conditions hardly interesting because:
1. there are no direct ways to increase the electrical conductivity during the process, which may be necessary for their use as carriers for semi-conductive elements;
2. the fineness of the powder, necessary so that it can be sintered at lower temperatures, requires a special treatment and the powder obtained is difficult to handle;
3. when using less fine powder (&gt;0.2 .mu.m ) the total shaped article has to be brought to the required high temperature; this is difficult to realize without the occurrence of local melting phenomena.
A process has now been found wherein the sintering can be carried out as well with coarser powder (&gt;0.2 .mu.m ) at lower temperatures in such a way that melting phenomena do not occur, wherein ordinary ovens employing Kanthal-winding (Kanthal is an iron alloy) can be used, wherein a fine porous structure, i.e., as fine as the starting powder, is obtained, and wherein practically no shrinkage occurs and the electrical conductivity increases.